IdentityManager to ApiServer - handling migration

Just wondering if anyone has good ideas for managing migration of the legacy Web Portal to the new Angular Portal. Version not a big player in this as its more of a generic question, but we are currently on 9.0 LTS CU3 for context.

With well over 10,000 requestables and several custom interfaces that we need to migrate, we expect this process to occur over an extended period. We are also trying to implement a bunch of initiatives that rely on the enhancements of the Angular Portal. Its unrealistic to expect a Big Bang implementation.

My Question is really more related to the requestables during the migration. We would have scenarios where we need certain requestables to only be available in the Old OR New portal, not both at the same time (especially where some of our initiatives require the modern Request Properties functionaltiy that only seems to work in Angular and is completely ignored in the Old portal which we can't have happening as users would be able to request products without supplying required infromation for processes to function). These are at the requestable level and wouldn't be segregated at the user level, so can't contemplate Shops with different Customer queries (and no way in the query to identify the portal that the user is accessing from).

Has anyone worked through this problem and come up with a solution or a list of possible solutions?